Avatar: The Last Airbender Season 2

Excellent

After leaving the North Pole and mastering Waterbending, Aang travels to the Earth Kingdom to master Earthbending. There, the group meets Toph, a blind Earthbending prodigy who becomes Aang's second teacher. The heroes discover information about an upcoming solar eclipse which would leave the Fire Nation powerless and open to invasion. They struggle to reach the Earth King with this vital information, but are detoured by Appa's kidnapping. Azula (Zuko's sister), and her two friends Mai and Ty Lee chase the group as they struggle to reach Ba Sing Se. Azula engineers a plan that allows the Fire Nation complete control over Ba Sing Se and the rest of the Earth Kingdom, and destroys any hope of a large-scale invasion of the Fire Nation. Azula is believed to have killed the Avatar using lightning,and there is no hope left.